Brake fluid doesn't evaporate if it's empty he has a major leak somewhere, I agree find the leak, repair, bleed system and hopefully all will be well....BUT the hard brake pedal is a bit confusing, I had my brakes fail, I had a full pedal and within 3 to 4 city blocks and using the brakes 3 or 4 times I went from the full pedal to no pedal or brakes at all, it was on Thanksgiving day 2 years ago 7:00pm and nothing was opened, I drove 90 miles home using down shifting and the parking brake when needed, made it home OK, I figure the master cylinder went out and at the time didn't know how these T4Rs brake system/master cylinder worked, thought it was the normal type system, I was very shocked to find out how much a master cylinder costs, popped the hood and saw I had no fluid so I then looked underneath and found my front left hard line was broken because the bolt holding it to the steering arm was snapped off and over time from just making turns the line broke from the small amount of flexing, replaced it bled the front brakes and all was good.
So him having a hard pedal seems to also point to another issue, but it could be hopefully just a leaking line or caliper.
